1. Pre-ground espresso. This causes all sorts of problems, the release of co2 being one of them (i.e. no crema!)
2. Too light of tamp/not enough volume. Way too fast of a pull means tons of extraction not taking place.
3. Letting the espresso sit attached to the group head before extraction. Burning the grounds does not taste good!
Sadly, it is usually the fourth "m" that causes bad coffee in Louisville.